1. Field of the Invention
The present invention relates to an image reading apparatus suitably used as an image sensor in a scanner, a copying machine, or the like.
2. Related Background Art
FIGS. 26 to 28 show an image sensor according to a related art. As shown in FIGS. 26 to 28, this sensor is constituted by a sensor unit 1, a sensor board 2 on which the sensor unit 1 is mounted, a lens array 3, an illumination device 4, a transparent glass plate 5, and a frame 6 for positioning/holding these components. The illumination device 4 includes lead frame type LEDs 7 and a light guide 8. An output from the sensor unit 1 is supplied to an external system through a connector 13 mounted on the sensor board 2. Note that halogen or xenon lamps or the like may be used as light sources in place of the LEDs.
The LEDs 7 are electrically connected to the external system through connector cables 16 constituted by lead portions 11 and LED connectors 12. As shown in FIG. 27, since the two LEDs 7 are arranged on both sides of the light guide 8, two connector cables 16 are required. In addition, since the LEDs 7 emit light onto the two ends of the light guide 8, the overall image sensor tends to increase in size in the longitudinal direction.
As shown in FIG. 28, light L emitted from the LED 7 is incident on an incident surface 8a of the light guide 8. The light having reached a diffusion portion 8d emerges from the light guide 8 through an exit surface 8c (FIG. 26). The light that is incident at an incident angle .theta. of 49.degree. or less (when the light guide is made of an acrylic resin and has a refractive index n=1.5) satisfies the total reflection angle condition and propagates in a desired direction.
When an original (not shown) is placed on the glass plate 5, the light emerging from the exit surface 8c passes through the glass plate 5 and is reflected by the original. The reflected light then reaches the sensor unit 1 through the lens array 3. The sensor unit 1 is a line sensor constituted by many photoelectric conversion elements arranged in a line, and serves to read an original image while scanning it. The read image signal is sent to the external system through the connector 13 and a lead wire.
A color image sensor according to the related art will be described next.
A light source switching type color image sensor has been known. This sensor includes LEDs respectively having the properties of emitting light beams of three colors, i.e., R, G, and B light beams. The sensor emits R, G, and B light beams at the same position on an original, and outputs signals by reading the reflected light beams. A color image signal corresponding to the original is then obtained in accordance with the output signals. FIGS. 29 to 33 show an example of the light source switching type color image sensor. This image sensor includes an LED array constituted by R, G, and B LEDs arranged in a line, a short-focus imaging element lens array, and a sensor array constituted by a plurality of line sensors arranged in a line.
Referring to FIGS. 29 and 30, a transparent glass plate 201 on which an original is to be placed is mounted on the upper portion of a frame 200. Light beams emitted form R, G, and B LEDs 230R1, 230G1, 230B1, 230R2, 230G2, 230B2, . . . , which are alternately arranged in a line on an LED board 210 mounted in the frame 200 as shown in FIG. 31, are reflected by the original placed on the upper surface of the glass plate 201. Reflected light beams 213 are read by a sensor array 10 on a board 19 through an optical system 209. As the optical system 209, the above short-focus image element lens array represented by, e.g., "SELFOC lens array" (available from Nippon Sheet Glass Co., Ltd.) is used.
As shown in FIG. 31, the LEDs 230R1, 230G1, 230B1, 230R2, 230G2, 230B2, . . . are mounted on the LED board 210. FIG. 32 shows the structure of each of these LEDs, and more specifically, the LED 230R1 as an example. An LED chip 211R1 is mounted on an LED base 216. The emission surface side of the LED chip 211R1 is covered with a transparent resin 215. On the LED board 210, these LEDs 230R1, 230G1, 230B1, 230R2, 230G2, 230B2, . . . can be ON/OFF-controlled at independent timings respectively set for R, G, and B.
As shown in FIG. 33, the sensor array 10 is constituted by a plurality of line sensors 10.sub.1, 10.sub.2, 10.sub.3, . . . arranged in a line on the board 19, and a protective film 206 covering the line sensors. In principle, the contact multichip image sensor is designed to form reflected light from an original into a one-to-one image on the sensor array 10 and read it. For this reason, the sensor array 10 needs to have a length equal to the width of an original to be read. Therefore, as the size of an original to be read changes, the required length of the sensor array 10 changes, and the number of line sensors constituting the sensor array 10 changes. Assume that an A3-size original is to be read. In this case, if one line sensor is 20 mm long, it suffices if the sensor array 10 is constituted by 15 line sensors 10.sub.1 to 10.sub.15.
The board 19 is supported by a bottom plate 205 engaged with the frame 200 as shown in FIG. 30, and connected to a flexible board 203 through a flexible interconnection 208 as shown in FIG. 33. A connector 202 for power, control signals, and the like is mounted on the flexible board 203. The flexible board 203 is fastened to the frame 200 with screws 207.
A color original read operation of the color image sensor having the above arrangement is started from loading of data for correction shading caused by variations in sensitivity of the respective line sensors or irregularities in light irradiated from the light sources. The shading correction data is loaded as follows. Light beams are sequentially emitted from the R LEDs 230R1, 230R2, . . . , the G LEDs 230G1, 230G2, . . . , and the B LEDs 230B1, 230B2, . . . in units of colors to read a white reference set in the image sensor. The resultant output signals from the image sensor are temporarily stored in a memory. Shading correction is performed by using the obtained R, G, and B shading correction signals obtained in such a manner that when the white reference is read again, the R, G, and B signals are uniform on one line, and an output signal r obtained when the R LEDs 211R1, 211R2, . . . are turned on, an output signal g obtained when the G LEDs 211G1, 211G2, . . . are turned on, and an output signal b obtained when the B LEDs 211B1, 211B2, . . . are turned on are set to r=g=b.
In an actual original read operation of the light source switching type color image sensor, to obtain R, G, and B signals at one point on the original to be read, R, G, and B light beams must be separately irradiated on the original. In this case, the operation of subscanning the image sensor over the entire original with one of the R, G, and B LEDs being turned on may be repeated three times while the type of LED turned on is changed. That is, the original may be read by the so-called field sequential scheme. Alternatively, the image sensor may be subscanned over the entire original while the R, G, and B LEDs are sequentially turned on in units of lines to be read, thereby obtaining R, G, and B signals. That is, the original may be read by the so-called line sequential scheme. By either of these methods, R, G, and B signals can be obtained from the entire original surface, and a color image can be reproduced by using the signals.
In the apparatus described with reference to FIGS. 26 to 28, however, the LEDs 7 are electrically connected to the external system through the connector cables 16, and the main irradiation direction of the LEDs 7 coincides with the longitudinal direction of the light guide 8. For this reason, the following problems are posed.
(1) Since the connector cables 16 equal in number to the LEDs 7 are required, the cost of the connector cables 16 and the number of steps of mounting them to the leads of the LEDs 7 increase, resulting in an increase in cost.
(2) The connector for inputting/outputting electrical signals between the image sensor and the external system and the connector for the light sources must be separately required. As a result, electrical connection to the external system becomes complicated, and the structure becomes susceptible to noise.
(3) Since no luminance adjustment circuits for the LEDs 7 can be arranged between the LEDs 7 and the external system, the illuminance on an original line to be read cannot be made uniform. For this reason, variations in sensor outputs (bright sensor outputs) obtained when image sensors read a white original become large among the image sensors.
(4) If a color image sensor for reading a color original by switching light sources for three colors (R, G, and B) is to be obtained by using the above arrangement shown in FIGS. 26 to 28, at least four lead portions 11 are required. Problems (1) to (3) described above become more serious.
(5) The structure including the portions required to hold the LEDs 7 on the two ends of the light guide 8 and to perform a process for the connector cables 16 becomes complicated, and the size in the longitudinal direction increases.
An LED array has many advantages over a tubular light source which has often used in the related art to illuminate a color original to read it. For example, the LED array is compact and has good response characteristics. Owing to these advantages, the LED array is expected as a color light source of the next generation. However, the following problems are posed in the light source switching type color image sensor using the R, G, and B LEDs described with reference to FIGS. 29 to 33.
(6) Since the types of LED chips are limited, the degree of freedom of illumination light is low. For this reason, optimal illumination light for a color original read operation cannot always be obtained.
(7) The spectral characteristics of the LED are greatly affected by changes in temperature, e.g., an increase in the temperature of the LED in a long-duration original read operation. For this reason, if the same color signal processing is always performed regardless of temperatures, color reproduction of an original image cannot be stably performed.
(8) If variations in spectral characteristics occur between a plurality of R, G, and B LED chips constituting an LED array, a color offset occurs on one original surface. It is therefore difficult to perform uniform color signal processing.